Comparing an outdated JavaScript library to a fundamental web technology doesn’t make sense to me at all.

If you work with the web in any serious way as a professional, AI or not, yes you should learn and understand CSS.

Vibe coders / Slopgineers that don't know the foundations of how these systems work are going to fall behind people who actually know how to program. Just because the machine can spit out text quickly doesn't mean it's good. And no, what all of these LLMs output isn't good css.